This invention relates in general to the field of materials technology and more specifically to the field of abrasive coatings for high temperature applications. In particular, the present invention pertains to an abrasive coating and a process for depositing that coating on component parts of a turbine combustion engine where the hard particles are co-deposited with a matrix material by means of a cold spraying process. Together, the hard particles and matrix material form an abrasive coating that provides a protective layer for the component parts so they are wear, erosion and abrasion resistant when used in high temperature environments such as a gas turbine.
It is well known that increasing the firing temperature in the combustion portions of a turbine may increase the power and operational efficiency of a gas turbine engine or a combined cycle power plant incorporating such a gas turbine engine. The demand for improved performance has resulted in advanced turbine designs wherein the peak combustion temperature may reach 1,400 degrees C. or more. Special materials are needed for components exposed to such temperatures. Nickel and cobalt based superalloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades. An example of a commercially available superalloy material is IN738 made by Inco Alloys International, Inc.
A metallic bond coat layer may be initially applied to the surface of a component to provide oxidation resistance and improved adhesion of an overlaying ceramic coating. Common metallic bond coat materials include MCrAlY and MCrAlRe, where M may be nickel, cobalt or iron or a mixture thereof. It is known in the art to apply the metallic bond coat layer by any one of several thermal spray processes, including low-pressure plasma spray (LPPS), air plasma spray (APS) and high velocity oxy-fuel (HVOF). Such processes propel the MCrAlY or MCrAlRe material, or other suitable materials, in a molten plasma state against the surface of the superalloy substrate where it cools and solidifies to form a coating. Such thermal spray processes are known to result in a significant amount of porosity and the formation of oxygen stringers in the metallic bond coat layer due to the inherent nature of a high temperature process. The release of heat from the molten particles of the metallic bonding materials and the transfer of heat from the high temperature gas used in a thermal spray process also result in a significant increase in the surface temperature of the superalloy substrate material during the metallic bond coat application process. Such elevated temperatures result in localized stresses in the superalloy material upon the cooling of the coating layer, which may have an adverse affect on the performance specifications of the superalloy component. Furthermore, a post-deposition diffusion heat treatment is necessary to provide the required metallurgical bond strength, and such treatment may also have adverse affects on the material properties of the underlying substrate.
To optimize the adhesion of the metallic bond coat to the superalloy substrate, it is desired to have a metal-to-metal contact between the layers. Any contamination, oxidation or corrosion existing on the surface of the substrate may adversely impact the adhesion of the coating layer. A separate cleaning step, such as grit blasting with alumina particles, is known in the art and may be used to clean the target surface. However, such process may leave trace amounts of the cleaning material on the surface. After even a short period of exposure to moisture in air, the target surface may begin to oxidize. Handling or storing of the component after the cleaning step may introduce additional contaminants to the previously clean surface. The environment of the prior art thermal spraying processes also contributes to the oxidation of the substrate during the coating process due to the presence of high temperature, oxygen and other chemicals. An improved process in the art is desirable to minimize the risk of oxidation during the application process.
It is also known in the art that the operational specifications of certain components within gas turbine engines require that hard particles abrade the coatings of other surfaces such as a turbine blade tip abrading the interior coating of a ring segment during operation. For example, U.S. Pat. No. 5,702,574 discloses a jig and the process by which the tip portion of a gas turbine blade is provided with hard particles embedded within a matrix material. The tip of the blade is designed to run against the inside surface of a blade encapsulating ring segment during operation of the gas turbine. As little clearance as possible is desired between the blade tips and the inside surface of the ring segment in order to minimize bypass flow of air and other gases past the tips of the blades. The material covering the inside surface of the ring segment is designed to be softer than the material on the blade tips so that as the abrasive material on the blade tips interacts with the interior surface of the ring segment, a very small gap is formed between the blade tips and the ring segment, which minimizes gas losses during operation of the turbine. In accordance with the ""574 patent, a plurality of blades may be mounted in a hollow jig having at least one ring of circumferentially disposed apertures through which the tips of the blades are inserted. The tips of the blades are then provided, by electrodeposition, with a coating of hard particles embedded within a matrix.
Electrodeposition is well known in the art and employed in the disclosure of U.S. Pat. No. 5,702,574 first identified above. For instance, the disclosed process includes situating the turbine blade tips within a jig such that they are encountered by a plating solution having hard particles entrained therein. As the particles encounter the tips they tend to settle on the tips where they become embedded in a metal that is being simultaneously plated out. This electrodeposition process, as well as other similar processes employing solutions such as electroplating or electroless plating, does not provide a means for precisely controlling the placement of abrasive particles on the blade tips, if desired.
Additionally, the invention disclosed in U.S. Pat. No. 5,702,574 includes deposition of an infill material by means of vibrating the jig assembly in order to coat regions of the blade tips that might otherwise be depleted of abrasive particles. Also, U.S. Pat. No. 5,076,897 discloses a similar vibration means used to plate infill of MCrAlY around abrasive particles deposited on portions of the blade tips. While electrodeposition and similar processes achieve good bonds they typically take several hours to perform and, in the case of depositing abrasive particles on the tips of turbine blades known in the art, must be performed in conjunction with rather elaborate apparatus that contribute to the cost of manufacture.
The known processes used to deposit abrasive particles within a matrix material on the tips of turbine blades, for example, have limitations such as they expose the underlying substrate to high temperatures, are time consuming, expensive and don""t necessarily achieve an optimum deposition of particles. The known apparatuses used in conjunction with these processes may be relatively elaborate and not easily adaptable for field repair, which increases the costs of manufacture or repair. Thus, an improved process is needed for depositing abrasive particles dispersed within a matrix material that will entrap the abrasive particles, sufficiently bond to a substrate, resist oxidation and possess sufficient mechanical properties to maintain its shape on the substrate.
The present invention uses a process, referred to herein as a cold spray process, to deposit hard particles that act as an abrasive onto a substrate to form an abrasive coating that is wear, erosion and abrasion resistant. The cold spray process may be used to co-deposit the hard particles with a matrix material to form a matrix composition on the substrate having the hard particles entrapped therein. The matrix material may be an MCrAlY composition or other suitable compositions provided the matrix material entraps the hard particles, forms a sufficient bond strength with the substrate, is resistant to high temperatures and oxidation, and has sufficient mechanical properties to maintain its shape on the substrate. The hard particles may be cubic boron nitride, diamond or other suitable particles having an appropriate level of hardness. The cold spray process may also be used to embed the hard particles directly into the superalloy substrate without the need for an accompanying matrix material.
One advantage of the present invention over the prior art methods of applying coatings using high temperature processes is that the substrate does not incur any damaging or debilitating effects often associated with high temperature coating applications. The cold spray process of the present invention may co-deposit the hard particles and matrix material in a low temperature environment, which prevents the substrate from suffering the adverse consequences such as altering heat-treated properties. Also, there is no need for a high temperature heat treatment following the deposition of the matrix material. As a result, the initial inter-diffusion zone between the substrate and matrix material is minimized. Further, the application of the matrix material using the cold spray process may be accomplished without masking, thereby eliminating process steps and eliminating the geometric discontinuity normally associated with the edge of a masked area. This feature also provides a cost savings advantage over prior art methods that require masking.
In one aspect of the present invention, the cold spray process allows for the co-deposition of a matrix material and hard particles on a wide range of substrates so that the hard particles are dispersed and entrapped within the matrix material. This process may be used with both new and service-run gas turbine components, for example. The co-deposition of the matrix material and hard particles may be effected by directing relative quantities of their constituent particles toward the substrate surface at a velocity sufficiently high to cause at least some of the matrix material particles to deform and to bond to the substrate surface while entrapping at least a portion of the hard particles within the matrix material to form a matrix composition on the substrate. The matrix composition forms an abrasive coating on the substrate. One advantage of the present invention is that the cold spray process may produce an abrasive coating having essentially no porosity and no oxygen stringers. These properties of the abrasive coating may increase its resistance to oxidation during operation, which is an improvement over known methods for applying coatings at high temperatures.
In one embodiment of the present invention, the depth of the matrix material may be varied along a surface of a substrate, so that a thicker coating is applied in those areas of the substrate exposed to the highest temperatures or those subject to higher incidence of rub encounters during operation, such as the tips of gas turbine blades rub encountering the inner surface of a ring segment during operation. Also, the composition of the matrix material may be varied along a surface of a substrate or across the depth of the matrix material if desired. This may be advantageous in that the consumption of an expensive material may be limited by applying it to only those portions of the substrate where the resulting benefit is necessary. Further, the composition of a first layer of the matrix material may be selected to minimize inter-diffusion with the underlying substrate material, and the composition of a second layer may be selected to optimize resistance to oxidation and corrosion.
Another advantage of the present invention is that the cold spray process permits the co-deposition of the matrix material and hard particles to be precisely controlled so that a layer or layers of hard particles may be dispersed within the matrix material, as the specific application requires. For instance, an exemplary embodiment of the present invention deposits an abrasive coating on the tips of gas turbine blades so that the hard particles are at their highest practical particle density per unit volume of the matrix material at or near the surface of the matrix material. This ensures a sufficient rub encounter with the interior surface of the ring segment during operation of the turbine. A high density of hard particles near the surface of the matrix material is desirable because the hard particles may oxidize over time, which may reduce the effectiveness of the abrasive coating. Varying the hard particle density per unit volume of matrix material across a gradient of layers may also extend the life cycle of the abrasive coating or achieve other performance requirements. Similarly, if desired, the cold spray process may be used with varying sizes of hard particles. Varying the size of the hard particles across the matrix material""s depth or along its surface may also prove to be advantageous depending on the specific application.
The cold spray process may also be used to deposit an initial layer of the matrix material on the surface of the substrate devoid or substantially devoid of hard particles then co-depositing the matrix material and hard particles to complete the abrasive coating. The initial layer of matrix material may increase the bond strength of the matrix material to the substrate and enhance oxidation resistance in that area. In one embodiment this initial layer has a depth approximately equal to the average diameter of the hard particles, which minimizes the likelihood that hard particles will inhibit the bond strength or adherence of the matrix material to the substrate. In an alternate embodiment, the initial layer of matrix material may be deposited first with the hard particles being deposited by themselves in a subsequent step. In this manner, the hard particles are directed at the previously deposited matrix material at a sufficient velocity so that they are embedded within the matrix material.
In another aspect of the present invention, the cold spray process may be used to directly deposit the hard particles onto the surface of a substrate without the need for a matrix material provided the composition of the substrate permits the hard particles to be embedded or entrapped therein. For example, a nickel base superalloy substrate, such as a gas turbine blade, may be sufficiently ductile to permit hard particles to be directly embedded into the substrate. If necessary, the substrate may be heated to within a specified temperature range prior to, during or after the deposition of the hard particles to ensure they are embedded and retained within the substrate.
Furthermore, the present invention takes advantage of the cold spray process to uniformly distribute the hard particles in the matrix material, which is desirable to achieve an even and predictable wearing of the abrasive coating. Providing a uniform distribution of particles helps to ensure they are sufficiently entrapped within the matrix material because the matrix material can substantially surround individual particles. It is, however, acceptable for particles to abut one or more other particles in which case the matrix material may surround adjoining particles. With known methods such as electrodeposition and electroplating or other solution bearing methods, for example, obtaining a uniform distribution of particles is difficult due to the inability to precisely control the particles"" deposition during the coating process. Uniformly depositing the hard particles within the matrix material on the tips of turbine blades also ensures a uniform and predictable rub encounter with the inner surface of a ring segment to effectuate a seal between the blade tips and the inner surface of a ring segment.
A further advantage of the present invention is that a desired halo effect of matrix material particles may be produced at the fringe of the cold spray area. In this aspect the particle speed of approach to the target surface is insufficient to cause the particles to bond to the surface of the substrate. Instead of bonding, the particles produce a desired grit blast/cleaning effect. This halo effect may be caused by the spread of particles away from a nozzle centerline due to particle interaction or by specific nozzle design. When the nozzle controlling application of the cold spray compound is directed perpendicular to the target surface the halo may be generally circular around a generally circular area being coated. The halo effect and cleaning action may also have an elliptical shape caused by a non-perpendicular angle between the nozzle centerline and the plane of the substrate target surface if so desired. The halo effect provides a cleaning of the target surface coincident to the application of the matrix material, which improves the adhesion of the coating when compared to prior art devices or methods where some impurities or oxidation may exist on the target surface at the time of material deposition.
Further, at least one embodiment of the present invention is sufficiently portable to permit the deposition of abrasive coatings in-situ, such as on the blades of a gas turbine while the blades are in the turbine at a power plant. This feature provides a significant cost savings relative to know methods that apply coatings with equipment fixed in place or that is otherwise too cumbersome or too costly to transport to remote sites. With this type of equipment the substrate to be treated, such as gas turbine blades requiring a replacement or supplemental coating, must be removed from its remote location and transported to the equipment site then back to its operational location and reinstalled.
These embodiments and advantages of the present invention are provided by way of example, not limitation, and are described more fully below.